The disclosure relates to a catadioptric projection objective including a plurality of optical elements arranged to image a pattern arranged in an object surface of the projection objective onto an image surface of the projection objective. The disclosure further relates to a projection exposure apparatus, a projection exposure method employing such projection objective, and a mirror.
Microlithographic processes are commonly used in the manufacture of semiconductor elements, such as integrated circuits (ICs) liquid crystal elements, micropatterned members and micromechanical components.
A projection exposure apparatus used for photolithography generally typically includes an illumination system configured to transform primary light from a light source into illumination light, and a projection objective. Light from the illumination system illuminates a reticle (or mask) having a given pattern, and the projection objective transfers an image of the pattern onto a region of a photo-sensitive substrate arranged in the image surface of the projection objective. Projection is typically performed with high resolution on a reduced scale to produce a demagnified image of the reticle pattern on the substrate.
In optical lithography, high resolution and good correction status of chromatic aberrations and other aberrations have to be obtained for a relatively large, virtually planar image field. Concave mirrors have been used for some time to help solve problems of chromatic correction and image flattening. A concave mirror has positive power, like a positive lens, but the opposite sign of Petzval curvature. Also, concave mirrors do not introduce chromatic aberrations. Therefore, catadioptric systems that combine refracting and reflecting elements, particularly lenses and at least one concave mirror, are often employed for configuring high-resolution projection objectives for microlithography using ultraviolet light.
Unfortunately, a concave mirror can be difficult to integrate into an optical design, since it sends the light right back in the direction it came from. Intelligent designs integrating concave mirrors are desirable.
Some catadioptric projection objectives allowing high image-side numerical aperture (NA) and a good correction status can include two or more cascaded (or concatenated) imaging objective parts and one or more intermediate images. One class of concatenated systems designed for use with off-axis fields to obtain an image free of vignetting and obscuration uses a single concave mirror positioned at or optically close to the pupil surface of a catadioptric objective part in combination with one or more negative lenses arranged ahead of the concave mirror to correct axial chromatic aberration (CHL) and Petzval sum. Typically, such projection objectives have a first deflecting mirror tilted relative to the optical axis, which mirror is used either to deflect the light coming from the object surface towards the concave mirror or in order to deflect the light reflected by the concave mirror toward downstream objective parts. A second deflecting mirror oriented at right angles with respect to the first deflecting mirror may be provided in order to parallelize the object plane and the image plane.
Representative examples for folded catadioptric projection objectives using planar deflecting mirrors in combination with a single concave mirror are disclosed, for example in US 2006/0077366 A1, US 2003/0234912 A1, US 2005/0248856 A1, US 2004/0233405 A1 or WO 2005/111689 A2.
In order to ensure that the deflecting mirrors have a high reflectivity, they are customarily coated with a reflective coating, usually designed as multiple dielectric layers (dielectric multilayer stack) or as a combination of metallic and dielectric layers. The reflectivity for light incident on and reflected by such mirrors is typically influenced in a polarization-dependent manner if dielectric layers are operated at high angles of incidence. Furthermore, as the image-side numerical aperture NA of projection objectives increases, the range of angles of incidence of light incident on a deflecting mirror (also denoted as spectrum of angles of incidence) may increase. For example, in projection objectives designed for immersion lithography at NA>1 angles of incidence given on a deflecting mirror tilted by 45° to the optical axis may range from about 30° to about 60°.
It would be desirable to have deflecting mirrors having reflective coatings with a high reflectivity (for example 90% or above) which is essentially constant for all angles of incidence occurring at the deflecting mirror and with a negligible polarization dependence of reflectivity. Using such “ideal” reflective coating could eliminate negative influence of the deflecting mirrors on the intensity distribution and polarization distribution of light within a projection beam in such catadioptric systems. However, such “ideal” reflective coatings are presently not available.
It has been found that, under certain imaging conditions in such catadioptric systems, structure lines having different orientation contained in the pattern to be imaged are projected with different efficiency. These orientation-dependent differences for various structure directions are also referred to as H−V differences (horizontal-vertical differences) or as variations in the critical dimensions (CD variations) and can be observed as differing line widths for the different structure directions in the photoresist.
In certain microlithography techniques, the pattern of the mask is illuminated with light from an effective source formed by an intensity distribution at a pupil plane of the illumination system corresponding to a particular illumination mode. The illumination modes include conventional illumination modes with various degrees of coherence (defined e.g. by parameter σ) and non-conventional illumination modes applying off-axis illumination. Off-axis illumination modes may be preferred when the critical dimensions desired for a particular lithography process are becoming very close to the theoretical resolution limit of the exposure system. With off-axis illumination, a mask providing a pattern is illuminated at oblique (non-perpendicular) angles, which may improve resolution, but particularly improves the process latitude by increasing the depth of focus (DOF) and/or contrast. One known off-axis illumination mode is annular, in which the conventional zero order spot on the optical axis is modified to a ring-shape intensity distribution. Another mode is multipole illumination, in which the intensity distribution at a pupil plane of the illumination system forming the effective source is characterized by several poles which are not on the optical axis (off-axis). Dipole illumination is frequently used for printing patterns having one predominant periodicity direction. Quadrupole illumination using four off-axis illumination poles may be used where a pattern contains sub-patterns of orthogonal lines along mutually perpendicular directions (sometimes denoted as horizontal and vertical lines).
Further improvements may be obtained by controlling the polarization state of the illumination light.